1. Field of the Invention
The present invention relates generally to voice and data communications systems and more specifically to such systems incorporating the integrated digital services network and earth-satellite-earth communications.
2. Description of the Prior Art
A telephone communications service, known as integrated service digital network (ISDN), is a communication network in which various services such as voice, data and facsimile (FAX) share the same transmission and switching facilities. A customer (subscriber) is presented with a common interface for all services and a unified set of control procedures are used to access the network. Common equipment is used to transmit and switch signals of all tile service varieties in and among network exchanges.
ISDN has mainly been considered as a terrestrial-based system. However, James S. Lee, in "Symbiosis Between a Terrestrial-Based Integrated Services Digital Network and a Digital Satellite Network," TUTORIAL: Integrated Services Digital Network (ISDN), by William Stallings, IEEE Computer Society, New York: 1983, pp. 302-308, suggests that digital satellite networks, such as the Satellite Business Systems (SBS) network and the French TELECOM I network, may be symbiotically combined with ISDN. Lee observes that satellite power and bandwidth are at a premium, therefore the efficient use of both is essential. The satellite motion in orbit results in a Doppler effect and the clock derived has some inherent frequency uncertainty that must be removed before it can be interfaced with a terrestrial digital network. The typicallone-way propagation of a satellite signal is one hundred and twenty milliseconds, which is relatively long, compared to terrestrial network delays.
Using existing satellite networks or creating proprietary networks causes interface and compatibility problems for users, especially small users who lack the resources necessary for the implementation of such networks. Thus for such users, there is a need for a system that can reliably carry ISDN channels over satellite communications links and present standard ISDN interfaces in the field to which ordinary and conventional equipment may be connected.